Conventional proton-exchange membrane fuel cell power generating equipment suitable as a small power supply typically comprises a reformer for reforming fuel gas, such as natural gas, town gas, methanol, liquefied petroleum gas (LPG), or butane, to hydrogen rich gas; a CO transformer for transforming carbon monoxide to CO2; CO removing apparatus for removing the carbon monoxide; a process gas burner for burning hydrogen until each reactor becomes stable during startup; a fuel cell for chemically reacting the hydrogen with oxygen from the air to generate power; a water tank for storing water that is treated by water treating apparatus using an ion-exchange resin or the like to cool an electrode part of the fuel cell and moisten the reaction air; a heat exchanger for recovering heat from exhaust gas from the reformer, the fuel cell, the process gas burner, or the like to produce hot water; and a hot water reserving tank for reserving the hot water.
A solid polymer electrolyte membrane used in such proton-exchange membrane fuel cell power generating equipment functions as a proton conductive electrolyte by containing water. The proton-exchange membrane fuel cell is operated by saturating the reaction air or reaction gas, such as fuel gas, with steam and supplying it to the electrode part.
When fuel gas containing hydrogen is fed to a fuel electrode and air is fed to an air electrode, a fuel electrode reaction for decomposing hydrogen molecules to hydrogen ions and electrons is performed in the fuel electrode, and an electrochemical reaction for generating water from oxygen with hydrogen ions in the air electrode occurs. Thus, the electrons moving through an external circuit from the fuel electrode to the air electrode carry power to a load and generate water on the air electrode side. FIG. 7 is a diagram of a conventional proton-exchange membrane fuel cell power generating equipment system (PEFC equipment GS). PEFC equipment GS typically includes an exhaust heat recovery device RD in addition to a fuel cell 6. The exhaust heat recovery device RD is coupled to a hot water reserving tank 50, heat exchanges 32, 46, 71, and pumps 33, 47, 72 through a hot water circuit or the like.
The fuel cell 6 has fuel gas feeding apparatus comprising a desulfurizer 2; a reformer 3; a CO transformer 4; CO removing apparatus 5 and the like; reaction air feeding apparatus comprising an air pump 11, a water tank 21 (distinct from the hot water reserving tank 50), and the like; electrodes, such as a fuel electrode 6a and an air electrode 6k; and cooling apparatus of the fuel cell 6, comprising the water tank 21, a pump 48, a cooling section 6c, and the like.
Power generated by the fuel cell 6 is increased in voltage by a direct current DC/DC converter (not shown) and is supplied to the commercial power supply via an electric distribution system cooperation inverter (not shown). The power is supplied from the power supply to houses or offices to be used for illumination or electric equipment, such as air conditioners.
The PEFC equipment GS uses the fuel cell 6 to generate power and uses heat generated at the same time to produce hot water from city water, accumulates the hot water in the hot water reserving tank 50, and supplies the hot water, such as for use in a bath or kitchen.
In the fuel gas feeding apparatus of the PEFC equipment GS, raw fuel 1, such as natural gas, town gas, methanol, LPG, or butane, is supplied to the desulfurizer 2, and here sulfur components are removed from the raw fuel. The raw fuel, having passed through the desulfurizer 2, is pressurized by a pressurizing pump 10 and supplied to the reformer 3. The raw fuel, while being supplied, is mixed with steam produced by feeding hot water from the water tank 21 through a water pump 22 and heating the hot water in a heat exchanger 17. The reformer 3 produces reformed gas containing hydrogen, carbon dioxide, and carbon monoxide. The reformed gas produced in the reformer 3 is supplied to the CO transformer 4, and here the carbon monoxide contained in the reformed gas is transformed to carbon dioxide. The gas from the CO transformer 4 is supplied to the CO removing apparatus 5. In the CO removing apparatus 5, untransformed carbon monoxide in the gas supplied from the CO transformer 4 is reduced to 10 ppm or less, and water gas (reformed gas) having a high hydrogen concentration is supplied to the fuel electrode 6a of the fuel cell 6 through a pipe 64. The amount of hot water supplied from the water tank 21 to the reformer 3 is adjusted to control moisture concentration in reformed gas.
In the reaction air feeding apparatus, air is fed from the air pump 11 to the water tank 21, and the reaction air is whipped in the hot water in the water tank 21 and is fed to a gas phase section 53, thereby moistening the reaction air. The air is moistened to facilitate the reaction in the fuel cell 6. The moistened reaction air is fed to the air electrode 6k of the fuel cell 6 from the water tank 21 through a pipe 25. The fuel cell 6 generates power by an electrochemical reaction of the hydrogen of the reformed gas fed to the fuel electrode 6a with oxygen in the air supplied to the air electrode 6k through the air pump 11 and the gas phase section 53 in the water tank 21.
The cooling apparatus of the fuel cell 6 is arranged along with the electrodes 6a, 6k of the fuel cell 6, and prevents the fuel cell 6 from being overheated by heat of the electrochemical reaction. The cooling apparatus circulates water from the water tank 21 as a coolant to a cooling section 6c with a pump 48, and the coolant maintains a proper temperature in the fuel cell 6 (for example, 70–80° C.) for the power generation.
The chemical reaction in the reformer 3 is an endothermic reaction, so that a burner 12 provides heat to the reformer 3 to maintain the chemical reaction. To the burner 12, raw fuel is supplied through a pipe 13, air is fed through a fan 14, and unreacted hydrogen is supplied from the fuel electrode 6a through a pipe 15. During startup of the PEFC equipment GS, the raw fuel is supplied through the pipe 13 to the burner 12. When the temperature of the fuel cell 6 becomes stable, the supply of the raw fuel through the pipe 13 is stopped, and, instead, the unreacted hydrogen (off-gas) discharged from the fuel electrode 6a is supplied through the pipe 15 to continue the combustion.
The chemical reactions performed in the CO transformer 4 and the CO removing apparatus 5 are exothermic reactions. During their operation, the CO transformer 4 and the CO removing apparatus 5 are cooled to prevent the CO transformer and the CO removing apparatus from reaching a reaction temperature. Predetermined chemical reactions and power generation occur in the reformer 3, the CO transformer 4, the CO removing apparatus 5, and the fuel cell 6.
Heat exchangers 18 and 19 are installed between the reformer 3 and the CO transformer 4, and between the CO transformer 4 and the CO removing apparatus 5, respectively. The water supplied from the water tank 21 circulates in the respective heat exchangers 18, 19 via pumps 23, 24, and cools respective gasses fed from the reformer 3 and the CO transformer 4. Another heat exchanger (not shown) may be also installed between the CO removing apparatus 5 and the fuel cell 6 to cool gas fed from the CO removing apparatus 5.
The heat exchanger 17 is connected to an exhaust system 31 of the reformer 3. When water is supplied from the water tank 21 via a pump 22, the heat exchanger 17 vaporizes the water to produce steam, and the steam mixes with the raw fuel and is fed to the reformer 3.
The PEFC equipment GS has a process gas burner (PG burner) 34. During-startup of the PEFC equipment GS, the composition of the reformed gas fed through the reformer 3, the CO transformer 4, and the CO removing apparatus 5 does not reach a defined stable value preferable for the operation of the fuel cell 6. Therefore, the gas cannot be fed to the fuel cell 6 until the composition becomes stable. Until each reactor becomes stable, the gas is guided to the PG burner 34 and burned in it. A fan 37 feeds air for combustion to the PG burner 34.
After each reactor becomes stable and the CO concentration in the gas reaches a defined value (for example, 10–20 ppm or lower), the gas is guided to the fuel cell 6 for power generation. Unreacted gas (off-gas) that cannot be used for power generation in the fuel cell 6 is initially guided to the PG burner 34 and burned, and, after the temperature of the fuel cell 6 becomes stable, the unreacted gas is guided to the burner 12 of the reformer 3 through a pipe 15.
Until each reactor becomes stable in temperature, on-off valve 91 is closed, and the reformed gas is fed to the PG burner 34 through the duct 35 and on-off valve 36. Even after the reactors become stable in temperature, until the temperature of the fuel cell 6 becomes stable in a range that is appropriate for producing electricity (for example, 70–80° C.), the on-off valve 91 is opened while the on-off valve 92 is closed, and the reformed gas is fed to the PG burner 34 through a duct 38 and an on-off valve 39, and the gas is burned in the PG burner. When the temperature of the fuel cell 6 becomes stable and appropriate for continuous power generation, both the on-off valve 91 and the on-off valve 92 are opened, while the on-off valve 36 and the on-off valve 39 are closed, and the unreacted gas (off-gas) is fed from the fuel cell 6 to the burner 12 through a duct 15.
City water is supplied to the hot water reserving tank 50 through an inlet 61. The water in the hot water reserving tank 50 is heated by exhaust heat generated from the PEFC equipment GS, and the heated water is supplied through the hot water supply pipe 62 to, for example, a kitchen, lavatory, or bath.
PEFC equipment typically includes several water circuits for recovering heat from exhaust gases. For example, heat is recovered from these exhaust gases and stored in the hot water reserving tank 50. Exhaust system 31 is connected to heat exchanger 32, in addition to heat exchanger 17, and the water in the hot water reserving tank 50 circulates in the heat exchanger 32 via pump 33 to recover heat from the exhaust gases passing through the exhaust system 31. Heat exchanger 46 is connected to exhaust system 45 of the PG burner 34, and the water in the hot water reserving tank 50 circulates in the heat exchanger 46 via pump 47 for exhaust heat recovery.
Heat is also recovered from exothermic chemical processes and stored in water tank 21. Water returned from the heat exchangers 18, 19 by pumps 23, 24, and coolant circulating in the cooling section 6c of the fuel cell 6, through water duct 73 and pump 48, flow into the water tank 21.
Water refilling apparatus 68 is connected to the water tank 21 to maintain a water level in the tank 21. The water refilling apparatus 68 includes an electromagnetic valve 56, a supply tank 67, and a pump 74. The supply tank 67 temporarily reserves water from city water refilling apparatus 69 and water recovered from the fuel cell 6 through a pipe 70. The supply tank 67 supplies the water to the water tank 21 as needed to maintain the water level in the tank 21. Water generated from the fuel cell 6 includes drain water obtained by a cooling system that includes the heat exchanger 71, the hot water reserving tank 50, circulating pump 72, as well as the water (condensate) contained in the gas exhausted from the fuel electrode 6a. 
The city water refilling apparatus 69 is connected to the water source 78 and includes an inlet 52 and an electromagnetic valve 76, which is opened by water level controller 77 when a water level gauge 79 detects an insufficient amount of water in the supply tank 67. The supply tank 67 is refilled through the inlet 52 and water treating apparatus (ion-exchange resin) 51, which uses the water pressure of the water source 78.
The water tank 21 includes a water level controller LC for keeping a water level sufficient to form an air section (gas phase section) 53 in the upper part of the tank and a temperature adjusting apparatus TC for keeping the temperature of the water in the tank 21 in a predetermined range. The water level controller LC includes a water level gauge 54 and an electromagnetic valve 56 for monitoring the water level in the water tank 21 and adding water as needed. Air passing through the water tank 21 is moisturized before being supplied to the fuel cell to facilitate the fuel cell reaction. The water level controller LC controls the water level so as to form the gas phase section 53 in the upper part of the water tank 21. When the water level decreases, the water level controller LC operates the pump 74, adjusts the opening of the electromagnetic valve 56 to feed treated water from the supply tank 67, through the pipe 84, into the water tank 21. The controller LC thus keeps the water level in the water tank 21 within the predetermined range.
A wave breaking plate 55 prevents the water level detection by the water level gauge 54 from being destabilized by foaming. The temperature control apparatus TC keeps the temperature of the water in the water tank 21 at a predetermined value or range, for example 60–80° C., so as to properly moisturize the reaction air supplied to the air electrode 6k of the fuel cell 6.
The water in the water tank 21 is heated by a heating device 63 installed in the water tank 21 if necessary.
The heat exchange between the water in the hot water reserving tank 50 and the heat exchanger in the fuel cell power generating equipment GS, as shown in FIG. 7, uses: a first circuit R1 between the tank and the first heat exchanger 32, through which combustion exhaust gas from the burner 12 of the reformer 3 passes; a second circuit R2 between the tank and the second heat exchanger 46 through which combustion exhaust gas from the PG burner 34 passes; and a third circuit R3 between the tank and a third heat exchanger 71 through which non-reacted oxygen gas exhausted from the air electrode of the fuel cell 6 passes. In other words, the heat exchange is performed between water fed from the hot water reserving tank 50 and the combustion exhaust gases or unreacted oxygen gas from the first heat exchanger 32, the second heat exchanger 46, and the third heat exchanger 71.
The PEFC equipment GS as discussed above is configured as a cogeneration system for power generation and heat utilization, so that the power generating efficiency of the fuel cell is relatively high and the water used in the system is effectively recycled.
However, when the hot water reserving tank 50 is full of hot water of a predetermined temperature, and the hot water cannot be discharged to the outside through the hot water supply pipe 62, additional exhaust heat from the gases cannot be recovered. Therefore, in order to keep the temperature of the coolant in the fuel cell 6 in a predetermined range, another cooling apparatus, such as a radiator (not shown), must be installed or operation of the equipment must be stopped. The installation of the cooling apparatus increases cost, and it makes reducing the size of the PEFC equipment GS difficult.
A layer of water at room temperature lies at the bottom of the hot water reserving tank 50, and a hot water layer with a lower density lies in the upper part of the tank. If no hot water is drawn from the hot water reserving tank 50 for an extended period of time while the system operates, the water in the bottom of the tank is heated by the heat exchanger and moves to the upper part, so that the hot water layer gradually increases and finally the tank is entirely filled with hot water.
When hot water is drawn from the hot water tank 50, hot water in the upper part is taken out to reduce the hot water layer, and city water is added into the bottom in proportion to the amount of hot water drawn off, resulting in an increase of the cooler water layer. Therefore, the temperature of the water fed from the bottom of the hot water reserving tank 50 to the heat exchangers is not constant, and heat exchanging efficiencies of the heat exchangers fluctuate. Furthermore, combustion exhaust gases passing through the three heat exchangers 32, 46, 71 contain gases having different temperatures. Consequently, the heat exchanging efficiencies fluctuate according to the temperature fluctuations of the water fed from the hot water reserving tank 50, resulting in low efficiency when the temperature difference is small.
In addition, if the water in the tank 21 decreases in temperature and freezes the water tank 21, the fuel cell 6, as well as piping lines and valves may be damaged, resulting in a malfunction of the equipment.